Ken- Terry's alluding to one of the SCR's attitudes... they'll trigger not only from a high breakover voltage (across A-C), but also from a fluctuation which causes the A-C relationship to swing enough to cause a differential voltage from GATE.
In the case of your DC motor controls, the DC motor can generate a substantial voltage and current from dynamic braking action, and from transients.
Consider the DC motor as being an ignition coil... the shunt and field windings around iron core, with a substantial field built up in motoring. Now, open the current path... what happens to the magnetic field? it collapses... right across it's own windings... WHAM! The voltage HAS to go somewhere, and it occurs EXTREMELY fast. The SCR's taking an awesome beating, and if it's surviving, it's quite a tough soldier.
What you SHOULD do, since you're not using speed-control, is just use a pair of contactors... one for braking, the other for running. If you really must use SCRs, at least put a low-value high-wattage resistor across the motor leads (always in-circuit) so that when you cut power to the motor, it'll drop SOME of it's field through the resistor (snubbing!) to keep from giving the SCRs a big Owie. A good WAG for resistance... take the resistance of the motor, and multiply that by 8... and use that resistance. Don't forget to apply Ohm's law to calculate current through the resistor, and from there, calculate wattage. Be very conservative, as the resistor WILL get warm.
If you want some contactors, I've still got a big pile of surplus in my shed, probably have some 400w shunt-resistors, too...